Lets say a page on
example.com has all of its images coming from
<img src="http://img.example.com/keyword.jpg" alt="keyword"/>
img.example.com disallows crawling:
User-agent: * Disallow: /
Would this make such images contribute less to a keyword value comparing to images from allowed domains?
I would guess so, because google probably would want to crawl an image url to see if anyone is using it on other sites, or if it even exists.